Method for the production of acrylic acid from propane, in the presence of molecular oxygen

ABSTRACT

The invention concerns the production of acrylic acid from propane in the presence of molecular oxygen. Said method consists in passing a gas mixture comprising propane, molecular oxygen, water vapour and, optionally, an inert gas, on a catalyst of formula (I) Mo l V a Te b Nb c Si d O x  to oxidize propane into acrylic acid, the propane/molecular oxygen mol ratio in the initial gas mixture being not less than 0.5.

The present invention relates to the production of acrylic acid frompropane in the presence of molecular oxygen.

It is known from European patent application No. EP-A-608838 to preparean unsaturated carboxylic acid from an alkane according to a catalyticoxidation reaction in vapour phase in the presence of a catalystcontaining a mixed metal oxide comprising as essential components, Mo,V, Te, O, as well as at least one element chosen from the groupconstituted by niobium, tantalum, tungsten, titanium, aluminium,zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium,nickel, palladium, platinum, antimony, bismuth, boron, indium andcerium, these elements being present in very precise proportions. Thereaction can be implemented using a gaseous mixture composed of thealkane, oxygen, an inert gas and water vapour corresponding to thefollowing molar proportions: alkane/oxygen/inert gas/watervapour=1/0.1-10/0-20/0.2-70 and preferably 1/1-5/0-10/5-40.

Moreover, the European patent application No. EP-A-895809 describescatalysts based on oxides comprising molybdenum, vanadium, niobium,oxygen, tellurium and/or antimony, as well as at least one other elementsuch as iron or aluminium. These catalysts can be used for theconversion of propane to acrylic acid, in the presence of molecularoxygen, as illustrated in Examples 9 and 10. Example 9, in particular,describes the oxidation of propane using a catalyst of formulaMo_(l)V_(0.33)Nb_(0.11)Te_(0.22)O_(n) from a gas flow composed ofpropane, oxygen, helium and a flow of water vapour, according to a molarratio propane/oxygen/helium/water vapour of approximately1/3.2/12.1/14.3. In such a gas flow, the flow of reactive gas has a verylow concentration of propane. Consequently the recycling of theunconverted propane is much more difficult because this unconvertedpropane is too diluted in the reaction flow.

The aim of the invention is to propose a method for the production ofacrylic acid from propane, in the presence of molecular oxygen, whichallows a higher conversion of propane to be obtained while retaininggood acrylic acid selectivity.

The inventors have discovered that this aim can be achieved by passing agaseous mixture of propane, oxygen and water vapour, and if appropriatean inert gas, over a particular catalyst, under conditions such that theoxygen of the gaseous mixture is in a substoichiometric proportion inrelation to the propane introduced, which probably allows the catalystto act in a similar way to a redox system and provides the oxygen whichis lacking so that the reaction is carried out in a satisfactory way.

The advantages of this novel method are the following:

-   -   the limitation of the overoxidation of the products formed which        takes place in the presence of too great a quantity of molecular        oxygen; according to the present invention, due to the fact of        operating in substoichiometry, the formation of CO_(X) (carbon        monoxide and carbon dioxide), degradation products, is reduced,        which allows the acrylic acid selectivity to be increased;    -   the acrylic acid selectivity is maintained at a good level;    -   the conversion is increased without loss of selectivity;    -   the catalyst undergoes only a low reduction and therefore a        small loss of its activity; it can easily be regenerated by        heating in the presence of oxygen or a gas containing oxygen        after a certain period of use; after regeneration, the catalyst        regains its initial activity and can be used in another reaction        cycle;    -   moreover, the separation of the stages of reduction of the        catalyst and of regeneration of the latter can be provided which        allows the partial pressure of propane to be increased, such a        partial supply pressure of propane being little limited by the        existence of an explosive zone created by the propane+oxygen        mixture, because the later is present in molecular form in        substoichiometric proportions;    -   moreover, this method allows reduction of the formation of        products produced by hydration, in particular propionic acid,        acetone and acetic acid.

The subject of the present invention is therefore a method for theproduction of acrylic acid from propane, in which a gaseous mixturecontaining propane, molecular oxygen, water vapour as well as, ifappropriate, an inert gas, is passed over a catalyst of formula (I):Mo_(l)V_(a)Te_(b)Nb_(c)Si_(d)O_(x)  (I)in which:

-   -   a is comprised between 0.006 and 1, inclusive;    -   b is comprised between 0.006 and 1, inclusive;    -   c is comprised between 0.006 and 1, inclusive;    -   d is comprised between 0 and 3.5, inclusive; and    -   x is the quantity of oxygen bound to the other elements and        depends on their oxidation state.        in order to oxidize the propane to acrylic acid, this method        being characterized in that the molar ratio propane/molecular        oxygen in the initial gaseous mixture is greater than or equal        to 0.5.

Such a method allows an acrylic acid selectivity to be obtained of closeto 60% and a high conversion of propane. Moreover, it can easily beimplemented in a fluidized bed or in a moving bed and the injection ofthe reagents can be carried out at different points of the reactor, soas to be outside of the flammability zone while having a high propaneconcentration and, consequently, a high catalyst productivity.

According to a particularly advantageous embodiment, the methodaccording to the invention comprises the following stages:

-   -   a) the gaseous mixture is introduced into a first reactor with a        moving catalyst bed,    -   b) at the outlet of the first reactor, the gases are separated        from the catalyst;    -   c) the catalyst is returned into a regenerator;    -   d) the gases are introduced into a second reactor with a moving        catalyst bed;    -   e) at the outlet of the second reactor, the gases are separated        from the catalyst and the acrylic acid contained in the        separated gases is recovered;    -   f) the catalyst is returned into the regenerator; and    -   g) the regenerated catalyst from the regenerator is reintroduced        into the first and second reactors;

According to another advantageous embodiment of the invention, thereactor or reactors are also provided with a cocatalyst. The abovestages can be carried out in a similar way using a catalyst and acocatalyst.

According to another advantageous embodiment of the invention, themethod comprises the repetition, in a reactor provided with the catalystof formula (I) and, if appropriate, with a cocatalyst, of the cyclecomprising the following successive stages:

-   1) a stage of injection of the gaseous mixture as defined above;-   2) a stage of injection of water vapour and, if appropriate of inert    gas;-   3) a stage of injection of a mixture of molecular oxygen, water    vapour and, if appropriate, inert gas; and-   4) a stage of injection of water vapour and, if appropriate inert    gas.

According to an improvement of the advantageous embodiment which hasjust been described, the cycle comprises an additional stage whichprecedes or follows stage 1) and during which a gaseous mixturecorresponding to that of stage 1) is injected but without the molecularoxygen, the molar ratio propane/molecular oxygen then being calculatedglobally for stage 1) and this additional stage.

According to an advantageous embodiment of the improvement which hasjust been presented, the additional stage precedes stage 1) in thecycle.

Other characteristics and advantages of the invention will now bedescribed in detail in the following description which is given withreference to the single attached FIGURE which diagrammaticallyrepresents an apparatus which is suitable for the implementation of anadvantageous embodiment of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, because the molar ratio propane/molecularoxygen in the initial gaseous mixture is greater than 0.5, theconversion of the propane to acrylic acid using the catalyst is carriedout by oxidation, probably according to the following concurrentreactions (1) and (2):

-   -   the standard catalytic reaction (1):        CH₃—CH₂-CH₃+2O₂→CH₂═CH—COOH+2H₂O  (1)    -   and the redox reaction (2):        SOLID_(oxidized)+CH₃—CH₂—CH₃→SOLID_(reduced)+CH₂═CH—COOH  (2)

The propane/water vapour volume ratio in the initial gaseous mixture isnot critical and can vary within wide limits.

Similarly, the proportion of inert gas, which can be helium, krypton, amixture of these two gasses, or nitrogen, carbon dioxide, etc., is alsonot critical and can also vary within wide limits.

The proportions of the constituents of the initial gaseous mixture aregenerally as follows (in molar ratios):propane/oxygen/inert(He—Kr)/H₂O (vapour)=1/0.05-2/1-10/1-10

Preferably, they are 1/0.1-1/1-5/1-5.

Yet more preferably, they are 1/0.167-0.667/2-5/2-5. As particularlybeneficial proportions the following may also be cited:

1/0.2-0.4/4-5/4-5.

Generally, reactions (1) and (2) are carried out at a temperature of 200to 500° C., preferably from 250 to 450° C., yet more preferably from 350to 400° C.

The pressure in the reactor or reactors is generally from 1.01×10⁴ to1.0×10⁶ Pa (0.1 to 10 atmospheres), preferably from 5.05×10⁴ to 5.05×10⁵Pa (0.5-5 atmospheres).

The residence time in the reactor, or if there are several, in eachreactor, is generally from 0.01 to 90 seconds, preferably from 0.1 to 30seconds.

The catalyst corresponds to the formula (I):Mo_(l)V_(a)Te_(b)Nb_(c)Si_(d)O_(x)  (I)in which:

-   a is comprised between 0.006 and 1, inclusive;-   b is comprised between 0.006 and 1, inclusive;-   c is comprised between 0.006 and 1, inclusive;-   d is comprised between 0 and 3.5, inclusive; and-   x is the quantity of oxygen bound to the other elements and depends    on their oxidation state.    Advantageously:-   a is comprised between 0.09 and 0.8, inclusive;-   b is comprised between 0.04 and 0.6, inclusive;-   c is comprised between 0.01 and 0.4, inclusive; and-   d is comprised between 0.4 and 1.6, inclusive.

The oxides of the different metals included in the composition of thecatalyst of formula (I) can be used as raw materials in the preparationof this catalyst, but the raw materials are not limited to the oxides;as other raw materials, there may be mentioned:

-   -   in the case of molybdenum, ammonium molybdate, ammonium        paramolybdate, ammonium heptamolybdate, molybdic acid,        molybdenum halides or oxyhalides such as MoCl₅, organometallic        compounds of molybdenum such as molybdenum alkoxides such as        Mo(OC₂H₅)₅, acetylacetone molybdenyl;    -   in the case of vanadium, ammonium metavanadate, vanadium halides        or oxyhalides such as VCl₄, VCl₅ or VOCl₃, organometallic        compounds of vanadium such as vanadium alkoxides such as        VO(OC₂H₅)₃;    -   in the case of tellurium, tellurium, telluric acid and TeO₂;    -   in the case of niobium, niobic acid, niobium tartrate, niobium        hydrogen oxalate, oxotrioxalatoammonium niobate        {(NH₄)₃[NbO(C₂O₄)₃ ]·1.5H₂O}, niobium and ammonium oxalate,        niobium oxalate and tartrate, nobium halides or oxyhalides such        as NbCl₃, NbCl₅ and organometallic compounds of niobium such as        niobium alkoxides such as Nb(OC₂H₅)₅, Nb(O-n-Bu)₅;        and, generally, all the compounds which are able to form an        oxide by calcination, namely, the metallic salts of organic        acids, the metallic salts of mineral acids, the metal complex        compounds, etc.        The source of silicon is generally constituted by colloidal        silica and/or polysilicic acid.

According to particular embodiments, the catalyst of formula (I) can beprepared by mixing aqueous solutions of niobic acid, ammoniumheptamolybdate, ammonium metavanadate, telluric acid under stirring, bythe addition preferably of colloidal silica, then by precalcinatingunder air between 280 and 340° C., preferably at approximately 320° C.and by calcinating under nitrogen at approximately 600° C.

Preferably, in the catalyst of formula (I):

-   a is comprised between 0.09 and 0.8, inclusive;-   b is comprised between 0.04 and 0.6, inclusive;-   c is comprised between 0.01 and 0.4, inclusive; and-   d is comprised between 0.4 and 1.6, inclusive.    Preferred Method for the Preparation of the Catalyst of Formula (I)

The catalyst of formula (I) can be prepared using a method in which:

-   1) a solution of niobium is prepared by mixing oxalic acid and    niobic acid, then optionally separation with a view to recovering    the solute;-   2) a solution of molybdenum, vanadium and tellurium is prepared;-   3) the two preceding solutions are coprecipitated by adding a silica    solution;-   4) the precipitate is dried, which produces a precursor with the    formula:    Mo_(l)V_(a)Te_(b)Nb_(c)Si_(d)(Oxalate)_(e)(NH₄)_(Y)O_(x).  (I′)

in which

a, b, c, d and x are defined as previously and

e is comprised between 0.006 and 3.5, inclusive;

y is comprised between 0.006 and 3.5, inclusive;

-   5) the precursor is precalcinated; then-   6) the precursor is calcinated.

The separation of stage 1) can be carried out by centrifugation,decantation or filtration.

The drying of stage 4) can be carried out in an oven in a thin layer, byatomization, freeze-drying, zeodration, with micro waves etc.

The precalcination can be carried out under air flow at 270-300° C. orunder static air at 320° C., in a fluidized bed, in a rotary tube, in aso-called aerated fixed bed, so that the catalyst pellets are separatedfrom each other in order to prevent them from fusing duringprecalcination or possibly during calcination.

The calcination is preferably carried out under very pure nitrogen andat a temperature less than 600° C., for example in a rotary furnace, ina fixed bed, in a fluidized bed and for a duration which can be 2 hours.

The catalyst obtained at the end of the calcination can be ground inorder to produce smaller particles. If the grinding is continued until apowder constituted by particles of approximately the size of a micron isobtained, the powder can subsequently be returned to its form using abinding agent such as for example silica in the form of polysilicicacid, the suspension then being dried again, for example by atomization.

According to a particularly preferred embodiment, the precalcination iscarried out

-   -   either at a temperature of approximately 275° C. under an air        flow of at least 10 ml/min/g of catalyst;    -   or at a temperature ranging from 300 to 350° C. under an air        flow less than 10 ml/min/g of catalyst.

According to a most particularly preferred embodiment, theprecalcination is carried out at approximately 320° C. under no airflow.

Regeneration of the Catalyst

During the redox reaction (2), the catalyst undergoes reduction and aprogressive loss of its activity. This is why, once the catalyst has atleast partially changed to the reduced state, its regeneration iscarried out according to reaction (3):SOLID_(reduced)+O₂→SOLID_(oxidized)  (3)by heating in the presence of oxygen or a gas containing oxygen at atemperature of 250 to 500° C., for a time necessary for the reoxidationof the catalyst.

The proportions of the constituents of the regeneration gaseous mixtureare generally as follows (in molar ratios):oxygen/inert(He—Kr)H₂O(vapour)=1/1-10/0-10

Preferably, they are 1/1-5/0-5.

Instead of using the oxygen alone, dry air (21% O₂) can be used. Insteadof or in addition to the water vapour, moist air can thus be used.

Generally the method is carried out until the reduction ratio of thecatalyst is comprised between 0.1 and 10 g of oxygen per kg of catalyst.

This reduction ratio can be monitored during the reaction through thequantity of products obtained. Then the equivalent quantity of oxygen iscalculated. It can also be monitored through the exothermicity of thereaction. The reduction ratio can also be monitored through the quantityof oxygen consumed in the regenerator.

After regeneration, which can be carried out under temperature andpressure conditions which are identical to, or different from those ofthe reactions (1) and (2), the catalyst regains an initial activity andcan be reintroduced into the reactors.

The reactions (1) and (2) and the regeneration (3) can be carried out ina standard reactor, such as a fixed bed reactor, a fluidized bed reactoror a moving bed reactor.

Thus the reactions (1) and (2) and the regeneration (3) can be carriedout in a device with two stages, namely a reactor and a regeneratorwhich operate simultaneously and in which two catalyst loadingsalternate periodically.

The reactions (1) and (2) and the regeneration (3) can also be carriedout in the same reactor by alternating the periods of reaction andregeneration.

Preferably, the reactions (1) and (2) and the regeneration (3) arecarried out in a reactor with a moving catalyst bed, in particular in avertical reactor, the catalyst then preferably moving from the bottomupwards.

A operating method with only one passage of the gas or with recycling ofthe gas can be used.

According to a preferred embodiment, the propylene produced and/or thepropane which has not reacted are recycled (or returned) to the inlet ofthe reactor, i.e. they are reintroduced at the inlet of the reactor, ina mixture or in parallel with the initial mixture of propane, watervapour and if appropriate inert gas or gases.

Use of an Apparatus with Two Reactors and a Regenerator

According to an advantageous embodiment of the invention, the methodaccording to the invention is used in an apparatus such as the onerepresented in the attached FIGURE.

The initial gaseous mixture comprising propane, molecular oxygen, watervapour as well as, if appropriate, an inert gas, is introduced into afirst reactor (Riser 1) containing the moving catalyst bed.

Then, at the outlet of the first reactor, the effluents are separatedinto gases and the moving bed catalyst.

The catalyst is sent into a regenerator.

The gases are introduced into a second reactor (Riser 2) also containinga moving catalyst bed.

At the outlet of the second reactor, the effluents are separated intogases and the moving bed catalyst.

The catalyst is sent into a regenerator.

The gases are treated in a known way, generally by absorption andpurification, with a view to recovering the acrylic acid produced.

The regenerated catalyst is reintroduced into the first reactor as wellas into the second reactor.

The method thus operates continuously, the circulation of the catalystbetween the reactors and the regenerator is carried out in a regular andgenerally continuous way.

Of course, the single regenerator can be replaced by two or moreregenerators.

Moreover, it is possible to add, after the second reactor, otherreactors which also have a catalyst circulating between each of thesereactors and the regenerator or other regenerators.

Preferably, the first and second reactors are vertical and the catalystis transported upwards by the gas flow.

A method of operating with only one passage of gases or with recyclingof the products leaving the second reactor can be used.

According to a preferred embodiment of the invention, after treatment ofthe gas originating from the second reactor, the propylene producedand/or the propane which has not reacted are recycled (or returned) tothe inlet of the reactor, i.e. they are reintroduced at the inlet of thefirst reactor, in a mixture or in parallel with the initial mixture ofpropane, oxygen, water vapour and if appropriate of inert gas or gases.

Use of a Cocatalyst

According to another advantageous embodiment of the invention, thegaseous mixture also passes over a cocatalyst.

This has the advantage of reducing the production of propionic acid,which is generally a by-product of the conversion reaction and whichposes problems in certain applications of acrylic acid when it ispresent in too great a quantity.

Thus, the propionic acid/acrylic acid ratio is greatly reduced at theoutlet of the reactor.

Moreover, the formation of acetone, which is also a by-product of theproduction of acrylic acid from propane, is reduced.

To this end, at least one of the reactors comprises a cocatalyst withthe following formula (II):Mo_(l)Bi_(a′)Fe_(b′)Co_(c′)Ni_(d′)K_(e′)Sb_(f′)Ti_(g′)Si_(h′)Ca_(l′)Nb_(j′)Te_(k′)Pb_(l′)W_(m′)Cu_(n′)  (II)in which:

-   -   a′ is comprised between 0.006 and 1, inclusive;    -   b′ is comprised between 0 and 3.5, inclusive;    -   c′ is comprised between 0 and 3.5, inclusive;    -   d′ is comprised between 0 and 3.5, inclusive;    -   e′ is comprised between 0 and 1, inclusive;    -   f′ is comprised between 0 and 1, inclusive;    -   g′ is comprised between 0 and 1, inclusive;    -   h′ is comprised between 0 and 3.5, inclusive;    -   i′ is comprised between 0 and 1, inclusive;    -   j′ is comprised between 0 and 1, inclusive;    -   k′ is comprised between 0 and 1, inclusive;    -   l′ is comprised between 0 and 1, inclusive;    -   m′ is comprised between 0 and 1, inclusive; and    -   n′ is comprised between 0 and 1, inclusive.

Such a cocatalyst can be prepared in the same way as the catalyst offormula (I).

The oxides of the different metals included in the composition of thecocatalyst of formula (II) can be used as raw materials in thepreparation of this cocatalyst, but the raw materials are not limited tothe oxides; as other raw materials, the corresponding nitrates can bementioned in the case of nickel, cobalt, bismuth, iron or potassium.

Generally, the cocatalyst is present in the form of a moving bed andpreferably it is regenerated and circulates, if appropriate, in the sameway as the catalyst.

Preferably, in the cocatalyst of formula (II):

-   -   a′ is comprised between 0.01 and 0.4, inclusive;    -   b′ is comprised between 0.2 and 1.6, inclusive;    -   c′ is comprised between 0.3 and 1.6, inclusive;    -   d′ is comprised between 0.1 and 0.6, inclusive;    -   e′ is comprised between 0.006 and 0.01, inclusive.    -   f′ is comprised between 0 and 0.4, inclusive;    -   g′ is comprised between 0 and 0.4, inclusive;    -   h′ is comprised between 0.01 and 1.6, inclusive;    -   i′ is comprised between 0 and 0.4, inclusive;    -   j′ is comprised between 0 and 0.4, inclusive;    -   k′ is comprised between 0 and 0.4, inclusive;    -   l′ is comprised between 0 and 0.4, inclusive;    -   m′ is comprised between 0 and 0.4, inclusive; and    -   n′ is comprised between 0 and 0.4, inclusive.

The weight ratio of the catalyst to the cocatalyst is generally greaterthan 0.5 and preferably at least 1.

Advantageously, the cocatalyst is present in the two reactors.

The catalyst and the cocatalyst are present in the form of solidcatalytic compositions.

They can each be in the form of pellets, generally of 20 to 300 μm indiameter, the catalyst and cocatalyst pellets generally being mixedbefore implementation of the method according to the invention.

The catalyst and the cocatalyst can also be present in the form of asolid catalytic composition composed of pellets each of which comprisesboth the catalyst and the cocatalyst.

EXAMPLES

The following examples illustrate the present invention without limitingits scope.

In the formulae given in Example 1, x is the quantity of oxygen bound tothe other elements and depends on their oxidation states.

The conversions, selectivities and yields are defined as follows:

${{Conversion}\mspace{14mu}(\%)\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{propane}} = {\frac{{Number}\mspace{14mu}{of}\mspace{11mu}{moles}{\;\;}{of}\mspace{14mu}{propane}\mspace{14mu}{having}\mspace{14mu}{reacted}}{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{introduced}} \times 100}$${{{Selectivity}{\mspace{11mu}\;}(\%)}\mspace{20mu}{for}{\mspace{11mu}\;}{acrylic}{\;\mspace{11mu}}{acid}} = {\frac{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{11mu}{of}\mspace{11mu}{acrylic}{\;\;}{acid}\mspace{14mu}{formed}}{{Number}{\;\;}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{having}{\mspace{11mu}\;}{reacted}} \times 100}$${{Yield}\mspace{14mu}(\%)\mspace{14mu}{of}{\mspace{11mu}\;}{acrylic}\mspace{14mu}{acid}} = {\frac{{Number}\mspace{14mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{acrylic}\mspace{11mu}{acid}\mspace{14mu}{formed}}{{Number}\mspace{11mu}{of}\mspace{14mu}{moles}\mspace{14mu}{of}\mspace{14mu}{propane}\mspace{14mu}{introduced}} \times 100}$The selectivities and yields relating to the other compounds arecalculated in a similar way.The conversion ratio is the weight of catalyst (in kg) required toconvert 1 kg of propane.

Example 1 Preparation of the Catalyst of FormulaMo_(l)V_(0.33)Nb_(0.11)Te_(0.22)Si_(0.95)O_(x)

a) Preparation of a Solution of Niobium

640 g of distilled water then 51.2 g of niobic acid (i.e. 0.304 moles ofniobium) are introduced into a 5 l beaker. Then 103.2 g (0.816 moles) ofdehydrated oxalic acid is added.

The molar ratio oxalic acid/niobium is therefore 2.69.

The solution obtained previously is heated at 60° C. for 2 hours, beingcovered so as to avoid evaporation and with stirring. Thus a whitesuspension is obtained which is left to cool to 30° C. under stirring,which takes approximately 2 hours.

b) Preparation of a Solution of Mo, V and Te

2120 g of distilled water, 488 g of ammonium heptamolybdate (i.e. 2.768moles of molybdenum), 106.4 g of ammonium metavanadate NH₄VO₃ (i.e.0.912 moles of vanadium) and 139.2 g of telluric acid (supplier: FLUKA)(i.e. 0.608 moles of tellurium) are introduced into a 5 l beaker.

The solution obtained previously is heated at 60° C. for 1 hour and 20minutes, being covered so as to avoid evaporation and with stirring. Inthis way a clear red solution is obtained which is left to cool to 30°C. under stirring, which takes approximately 2 hours.

c) Introduction of the Silica

393.6 g of Ludox silica (containing 40% by weight of silica, supplied byDupont) is introduced under stirring into the previously preparedsolution of Mo, V and Te. The latter retains its limpidity and its redcolouring.

Then the previously prepared solution of niobium is added. In this way afluorescent orange gel is obtained after stirring for a few minutes.This solution is then dried by atomization. The atomizer used is alaboratory atomizer (ATSELAB from Sodeva). The atomization takes placein a nitrogen atmosphere.

The working parameters are globally:

-   -   flow rate of nitrogen of the order of 45 Nm³/h;    -   flow rate of slurry of the order of 500 g/h;    -   inlet temperature of the gas comprised between 155° C. and 170°        C.;    -   outlet temperature of the gas comprised between 92° C. and 100°        C.

Then the product recovered (355.2 g), which has a particle size lessthan 40 microns, is placed in an oven overnight at 130° C., in ateflon-covered plate.

In this way 331 g of dry product is obtained.

d) Calcination

The precalcinations and calcinations were carried out under air andnitrogen flow in steel capacitors. These capacitors are directlyinstalled in muffle furnaces and the air is supplied via the flue. Aninternal thermometer well allows precise monitoring of the temperature.The cover is useful to prevent air returning towards the catalyst.

Firstly, the 331 g of precursor obtained previously are precalcinatedfor 4 hours at 300° C. under air flow of 47.9 ml/min/g of precursor.

The solid obtained is then calcinated for 2 hours at 600° C. under anitrogen flow of 12.8 ml/min/g of solid.

In this way the desired catalyst is obtained.

Example 2 Catalyst Tests

a) Apparatus

In order to simulate the method according to the invention, simulationswere carried out in a laboratory fixed bed reactor, by generatingpropane pulses and oxygen pulses.

The following are loaded from the bottom to the top of a verticalreactor with cylindrical shape and made of pyrex:

-   -   a first height of 1 ml of silicon carbide in the form of        particles of 0.125 mm in diameter,    -   a second height of 1 ml of silicon carbide in the form of        particles of 0.062 mm in diameter,    -   a third height of 5 g of catalyst in the form of particles of        0.02 to 1 mm diluted with 10 ml of silicon carbide in the form        of particles of 0.062 mm in diameter,    -   a fourth height of 1 ml of silicon carbide in the form of        particles of 0.062 mm in diameter,    -   a fifth height of 3 ml of silicon carbide in the form of        particles of 0.125 mm in diameter and    -   a sixth height of silicon carbide in the form of particles of        1.19 mm in diameter so as to fill all of the reactor.

b) Operating Method

The reactor is then heated to 250° C. and the vaporiser to 200° C. Theelectric initiation of the water pump is actuated.

Once the reactor and the vaporiser have reached the temperatures givenabove, the water pump is actuated and the temperature of the reactor israised to the desired test temperature, i.e. 400° C.

The hot spot of the reactor is then left to stabilize for 30 minutes.

Then, oxygen is introduced in 10 pulses of 23 seconds each in order tosufficiently oxidize the catalyst. The catalyst is considered to betotally oxidized when the temperature of the hot spot has stabilized,i.e. when there is no more exothermal activity due to the reaction (bymonitoring the catalyst temperature measured using a thermocouple placedin the catalyst bed, the fluctuations in temperature can be seen as afunction of the pulses).

The pressure at the inlet of the reactor was approximately 1.1 to 1.8bars (absolute) and the pressure drop across the reactor isapproximately 0.1 to 0.8 bars (relative).

Test A

-   -   The production of acrylic acid was measured using a redox        balance.    -   A redox balance is composed of 40 redox cycles. A redox cycle        represents:    -   10 cycles of:    -   30 seconds of propane+5 seconds of oxygen (oxygen being injected        at the start of the propane pulse), with the proportions        propane/O₂/He—Kr/H₂O of 10/10/45/45, with a helium-krypton flow        of 4.292 N1/h (N1=liter of gas at 0° C. and 760 mm Hg);    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 45 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 45 seconds;    -   10 cycles of:    -   30 seconds of propane+10 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 10/10/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 45 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 45 seconds;    -   10 cycles of:    -   30 seconds of propane+15 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 10/10/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 45 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 45 seconds; and    -   10 cycles of:    -   30 seconds of propane+20 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 10/10/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 45 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 45 seconds.

During the balancing, four samples are taken, each representing 10cycles. 4 samples of gas are also carried out using gas bags, eachsample representing approximately 10 cycles. (The gas samples arecarried out over a period corresponding to a multiple of the duration ofa cycle, in order to be able to know the theoretical quantity of propaneinjected).

Each small gas-washing bottle (with a 25 ml capacity and filled with 20ml of water) is equipped with a gas bag, and when the bottle isconnected to the outlet of the reactor (as soon as the liquid bubbles),the bag is open and the chronometer is started.

In order to verify the oxidation state of the catalyst, another seriesof ten 23-second pulses of oxygen is carried out. It shows that theoxidation state of the solid has been maintained during the balancing(no exothermal activity).

The liquid effluents are analyzed on a BP 6890 chromatograph, afterhaving carried out a specific calibration.

The gases are analyzed during the balancing on a Chrompack micro-GCchromatograph.

An assay of the acidity is carried out on each bottle during theoperation, in order to determine the exact number of moles of acidproduced and to validate the chromatographic analyses.

Test B

The procedure was undertaken as in test A, except that the redox balancewas composed of the following 40 redox cycles:

-   -   10 cycles of:    -   30 seconds of propane+5 seconds of oxygen, (the oxygen being        injected at the start of the propane pulse) with the proportions        propane/O₂/He—Kr/H₂O of 20/15/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;    -   10 cycles of:    -   30 seconds of propane+10 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 20/15/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;    -   10 cycles of:    -   30 seconds of propane+15 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 20/15/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds; and    -   10 cycles of:    -   30 seconds of propane+20 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 20/15/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;        Test C

The procedure was the same as in test A, except that the redox balancewas composed of the following 40 redox cycles:

-   -   10 cycles of:    -   30 seconds of propane+5 seconds of oxygen, (the oxygen being        injected at the start of the propane pulse) with the proportions        propane/O₂/He—Kr/H₂O of 20/20/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;    -   10 cycles of:    -   30 seconds of propane+10 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 20/10/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;    -   10 cycles of:    -   30 seconds of propane+15 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 20/6.7/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds; and    -   10 cycles of:    -   30 seconds of propane+20 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 20/5/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds.        Test D

The procedure was the same as in test A, except that the redox balancewas composed of the following 40 redox cycles:

-   -   10 cycles of:    -   30 seconds of propane+5 seconds of oxygen, (the oxygen being        injected at the start of the propane pulse) with the proportions        propane/O₂/He—Kr/H₂O of 30/30/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;    -   10 cycles of:    -   30 seconds of propane+10 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 30/15/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;    -   10 cycles of:    -   30 seconds of propane+15 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 30/10/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds; and    -   10 cycles of:    -   30 seconds of propane+20 seconds of oxygen, with the proportions        propane/O₂/He—Kr/H₂O of 30/7.5/45/45, with a flow of        helium-krypton of 4.292 N1/h;    -   an intermediate pulse composed only of the flow of carrier gas        He—Kr/H₂O of 60 seconds;    -   an oxygen pulse with the proportions O₂/He—Kr/H₂O=20/45/45 for        60 seconds and    -   another intermediate pulse composed only of the flow of carrier        gas He—Kr/H₂O of 60 seconds;        c) Results

The final results correspond to the microbalances carried out on the 4gas-washing bottles and the 4 gas bags.

In tests A and B, the quantity of oxygen injected was increasing whenchanging from one series of 10 cycles to another, because the durationof the oxygen pulse was increasing.

In tests C and D, the quantity of oxygen remained constant when changingfrom one series of 10 cycles to another. In fact, although the durationof the oxygen pulse increased form one series of 10 cycles to another,the proportion of oxygen in the pulse was adjusted (reduced) each time.

The results are compiled in the following Tables I and II:

TABLE I TEST A B C D In the reaction: 10 + 10/45/45 20 + 15/45/45 20 +(20/10/6.7/5)45/45 30 + (30/15/10/7.5)/45/45 propane + oxygen/ He—Kr/H₂OSelectivities (%) Duration of the oxygen 5 10 15 20 5 10 15 20 5 10 1520 5 10 15 20 pulses injected into the propane pulses Acrylic acid 45.248.6 50.1 56.2 39.9 41.6 45.8 49.0 35.0 35.9 31.7 28.2 33.9 35.1 29.128.6 Acetic acid 10.7 8.9 10.2 8.1 10.9 9.9 8.7 8.9 11.9 12.2 13.1 13.612.8 12.9 14.8 16.0 Acrolein 0.16 0.15 0.15 0.15 0.12 0.10 0.10 0.100.08 0.08 0.08 0.08 0.09 0.09 0.09 0.09 Acetone 0.58 0.53 0.52 0.51 0.810.67 0.58 0.54 0.78 0.74 0.81 0.78 0.96 1.00 1.14 1.24 Propionic acid0.24 0.28 0.28 0.25 0.22 0.22 0.21 0.20 0.21 0.96 0.22 0.21 0.33 0.660.23 0.25 Allyl alcohol 0.04 0.06 0.08 0.06 0.03 0.03 0.03 0.04 0.020.02 0.02 0.02 0.01 0.02 0.02 0.02 Allyl acrylate 0.00 0.09 0.10 0.110.04 0.10 0.04 0.10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Propylaldehyde 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Acetaldehyde 0.05 0.00 0.00 0.00 0.04 0.04 0.03 0.030.03 0.03 0.03 0.03 0.03 0.03 0.04 0.04 CO 15.3 15.1 14.4 12.8 16.1 16.115.6 15.1 17.7 16.8 18.2 19.0 15.6 15.0 16.4 15.7 CO₂ 15.9 14.7 12.811.4 18.1 17.1 15.4 13.5 20.0 19.2 21.0 23.1 18.3 17.8 20.0 19.9Propylene 11.8 11.6 11.4 10.4 13.7 14.2 13.5 12.4 14.2 14.1 14.8 15.118.0 17.3 18.3 18.1 Selectivity in acrylic 45.2 48.6 50.1 56.2 39.9 41.645.8 49.0 35.0 35.9 31.7 28.2 33.9 35.1 29.1 28.6 acid Selectivity inacrylic 57.0 60.2 61.5 66.6 53.5 55.8 59.2 61.4 49.2 50.0 46.6 43.3 51.952.5 47.3 46.7 acid and propylene

TABLE II TEST A B In the reaction: 10 + 10/45/45 20 + 15/45/45 propane +oxygen/ He—Kr/H₂O Selectivities (%) Duration of the oxygen 5 10 15 20 510 15 20 pulses injected into the propane pulses Yields (%) Acrylic acid11.75 13.05 14.04 16.73 9.14 9.54 11.16 12.99 Acetic acid 2.77 2.40 2.852.41 2.51 2.26 2.13 2.37 Acrolein 0.04 0.04 0.04 0.05 0.03 0.02 0.020.03 Acetone 0.15 0.14 0.15 0.15 0.19 0.15 0.14 0.14 Propionic acid 0.060.07 0.08 0.07 0.05 0.05 0.05 0.05 Allyl alcohol 0.01 0.02 0.02 0.020.01 0.01 0.01 0.01 Allyl acrylate 0.00 0.02 0.03 0.03 0.01 0.02 0.010.03 Propyl aldehyde 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Acetaldehyde 0.01 0.00 0.00 0.00 0.01 0.01 0.01 0.01 CO 3.98 4.06 4.043.82 3.70 3.68 3.80 4.01 CO₂ 4.14 3.94 3.59 3.41 4.15 3.93 3.76 3.58Propylene 3.06 3.12 3.19 3.09 3.13 3.25 3.28 3.28 Propane 73.55 72.6571.76 69.72 77.20 77.14 75.55 73.59 Carbon balance (%) 99.53 99.52 99.7999.49 100.12 100.09 99.92 100.09 Quantity of oxygen 1.64 1.73 1.79 1.882.77 2.85 2.97 3.18 consumed (g O/kg catalyst) μmole propane for 1 390406 412 417 735 767 767 769 cycle μmole O₂ added per 53 107 160 214 107214 320 427 cycle μmole O consumed 511 542 559 589 865 891 929 994(products formed)/cycle Propane conversion 1058 1023 991 924 650 649 606561 ratio (kg catalyst/kg propane converted) TEST C D In the reaction:20 + (20/10/6.7/5)45/45 30 + (30/15/10/7.5)/45/45 propane + oxygen/He—Kr/H₂O Selectivities (%) Duration of the oxygen 5 10 15 20 5 10 15 20pulses injected into the propane pulses Yields (%) Acrylic acid 7.848.15 6.95 6.07 6.55 7.06 5.59 5.52 Acetic acid 2.67 2.76 2.87 2.92 2.482.59 2.84 3.10 Acrolein 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Acetone0.17 0.17 0.18 0.17 0.19 0.20 0.22 0.24 Propionic acid 0.05 0.22 0.050.05 0.06 0.13 0.05 0.05 Allyl alcohol 0.00 0.00 0.00 0.01 0.00 0.000.00 0.00 Allyl acrylate 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Propylaldehyde 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Acetaldehyde 0.01 0.010.01 0.01 0.01 0.01 0.01 0.01 CO 3.96 3.81 3.98 4.08 3.00 3.02 3.15 3.04CO₂ 4.48 4.37 4.60 4.96 3.53 3.58 3.84 3.85 Propylene 3.18 3.20 3.253.26 3.47 3.49 3.51 3.50 Propane 77.69 77.38 78.21 78.44 80.63 79.8980.88 80.73 Carbon balance (%) 100.09 100.10 100.10 99.98 99.93 99.99100.11 100.06 Quantity of oxygen 2.77 2.84 2.85 2.89 3.59 3.79 3.71 3.70consumed (g O/kg catalyst) μmole propane for 1 732 752 762 764 1162 11841178 1175 cycle μmole O₂ added per 107 107 107 107 161 161 161 161 cycleμmole O consumed 867 889 892 902 1122 1184 1158 1157 (productsformed)/cycle Propane conversion 678 669 694 701 495 477 502 498 ratio(kg catalyst/kg propane converted)From Table I above the following observations can be drawn:

-   -   in the same test A or B, the larger the quantity of oxygen        injected, the higher the acrylic acid selectivity;    -   tests C and D show that it is better to have a high partial        pressure of oxygen for a short time than the same quantity of        oxygen for a longer time.        From Table II the following observations can be drawn:    -   the quantity of oxygen consumed, calculated on the basis of the        products formed, does not increase much with the addition of        molecular oxygen and the conversion does not change much with        the addition of molecular oxygen;    -   the quantity of oxygen consumed (in μmoles of atomic oxygen) is        greater than the quantity added by the pulse; this means that        the catalyst was reduced in any case;    -   the conversion is higher in Test A;    -   the lowest conversion ratio is obtained when the partial        pressure of propane is highest.

Example 3 Comparative

A catalyst was prepared in the following way.

5.35 g of ammonium paramolybdate and 0.80 g of tellurium dioxide (TeO₂)are successively added under stirring into 20 ml of water. Separately, asolution containing 15 mmoles of vanadium is prepared by dissolving 3.94g of hydrated vanadyl sulphate in 20 ml of distilled water. The twosolutions are then slowly mixed and stirred for 10 minutes before beingintroduced into a 70 ml autoclave covered with Teflon®. Then nitrogen isbubbled through for 5 minutes so that it substitutes the air present inthe autoclave, before the latter is closed. The autoclave is then set at175° C. for 72 hours.After this period, the autoclave is cooled with water under the tap for10 minutes. The black solid obtained in the autoclave is separated fromthe solution by filtration, thoroughly washed with distilled water anddried for 12 hours at 80° C. The precursor thus obtained is thenprecalcinated under air at 280° C. for 2 hours. Then, the solid obtainedis calcinated under nitrogen flow (25 ml/h/g) at 600° C. for 2 hours. Inthis way catalyst 3 is obtained.Test Conditions

500 mg of catalyst 3 are vigorously ground and poured into a reactormade of Pyrex®. The selective oxidation reaction for propane is carriedout at atmospheric pressure, in a reactor under a conventional flow. Thegaseous reaction mixture propane/oxygen/nitrogen/water vapour(6.5%/10%/38%/45%) is introduced into the reactor maintaining a contacttime of 2.1 s. The catalyst is tested at 320 and 360° C. The results arecompiled in Tables 2 and 3.

Example 4

A catalyst was prepared in the following way.

4.32 g of molybdenum oxide, 0.80 g of tellurium dioxide (TeO₂) and 0.82g of vanadium oxide V₂O₅ are added successively and under stirring to 30ml of water heated to 80° C. Separately, a solution containing 3.6mmoles of niobium is prepared by dissolving under stirring, 2.33 g ofhydrated niobium oxalate in 10 ml of distilled water heated to 80° C.The two solutions are then slowly mixed and stirred for 10 minutesbefore being introduced into a 70 ml autoclave covered with Teflon®.Then nitrogen is bubbled through for 5 minutes so that it replaces theair present in the autoclave, before the latter is closed. The autoclaveis then set at 175° C. for 48 hours.

After this time, the autoclave is cooled down with tap water, for 10minutes. The black-blue solid obtained in the autoclave is separatedfrom the solution by filtration, washed thoroughly with distilled waterand dried for 12 hours at 80° C. The precursor thus obtained is thencalcinated under nitrogen flow (25 ml/h/g) at 600° C. for 2 hours. Inthis way catalyst 4 is obtained. This catalyst is tested under the sameconditions as catalyst 3. The results are compiled in Tables 2 and 3.

Example 5

A catalyst was prepared in the following way.

5.35 g of ammonium paramolybdate and 1.16 g of telluric acid aresuccessively added to 20 ml of water heated to 80° C. under stirring.Separately, a solution containing 9 mmoles of vanadium is prepared bydissolving 2.37 g of hydrated vanadyl sulphate in 10 ml of distilledwater heated to 80° C. A third solution containing 3.6 mmole of niobiumis simultaneously prepared by dissolving under stirring, 2.33 g ofhydrated niobium oxalate in 10 ml of distilled water heated to 80° C.The second solution is added to the first and the mixture is stirred for5 minutes. Finally the third solution is added and the mixture is thenstirred for 10 minutes before being introduced into a 70 ml autoclavecovered with Teflon®. Then nitrogen is bubbled through for 5 minutes sothat it substitutes the air present in the autoclave, before the latteris closed. The autoclave is then set at 175° C. for 48 hours.

After this time, the autoclave is cooled down with tap water, for 10minutes. The black-blue solid obtained in the autoclave is separatedfrom the solution by filtration, washed thoroughly with distilled waterand dried for 12 hours at 80° C. The precursor thus obtained is thencalcinated under nitrogen flow (25 ml/h/g) at 600° C. for 2 hours. Inthis way catalyst 5 is obtained. This catalyst is tested under the sameconditions as catalyst 3. The results are compiled in Tables 2 and 3.

Example 6

A catalyst was prepared in the following way.

5.35 g of ammonium paramolybdate is added to 20 ml of water heated to80° C. under stirring. Separately, a solution containing 15 mmoles ofvanadium is prepared by dissolving 3.94 g of hydrated vanadyl sulphatein 20 ml of distilled water heated to 80° C. The second solution isadded to the first and the mixture is then stirred for 10 minutes beforebeing introduced into a 70 ml autoclave covered with Teflon®. Thennitrogen is bubbled through for 5 minutes so that it substitutes the airpresent in the autoclave, before the latter is closed. The autoclave isthen set at 175° C. for 24 hours.

After this time, the autoclave is cooled down with tap water, for 10minutes. The black-blue solid obtained in the autoclave is separatedfrom the solution by filtration, washed thoroughly with distilled waterand dried for 12 hours at 80° C. The precursor thus obtained is thencalcinated under nitrogen flow (25 ml/h/g) at 500° C. for 2 hours. Inthis way catalyst 6 is obtained. This catalyst is tested under the sameconditions as catalyst 3.

According to a similar operating method, tellurium can be introducedinto this preparation.

TABLE 1 Summary table of the different preparations Composition of theExample No. solution (without oxygen) Method of preparation Example 3Mo_(1.0)V_(0.50)Te_(0.16) Hydrothermal synthesis Example 4Mo_(1.0)V_(0.50)Te_(0.16)Nb_(0.12) Hydrothermal synthesis Example 5Mo_(1.0)V_(0.30)Te_(0.16)Nb_(0.12) Hydrothermal synthesis Example 6Mo_(1.0)V_(0.50) Hydrothermal synthesis

TABLE 2 Oxidation of propane at 320° C. on tellurium catalystsConversion (%) Ex- am- Selectivity (%) Yield (%) ple Acrylic AceticAcrylic No. C₃H₈ acid C₃H₆ Acetone acid CO CO₂ acid 3 17.1 50.1 13.56.00 13.6 9.25 7.55 7.72 4 15.7 62.8 19.4 2.47 6.65 5.20 3.47 9.86 516.2 59.5 16.1 2.77 9.57 7.20 4.88 9.62 6 11.1 5.41 19.5 0.97 20.4 33.120.6 0.60

TABLE 3 Oxidation of propane at 360° C. on tellurium catalystsConversion (%) Ex- Yield am- Selectivity (%) (%) ple Acrylic AceticAcrylic No. C₃H₈ acid C₃H₆ Acetone acid CO CO₂ acid 3 36.2 47.3 7.491.33 16.7 14.0 13.3 16.3 4 25.5 69.0 8.83 0.40 5.03 9.22 7.52 24.5 533.4 62.4 8.72 0.45 7.27 11.1 10.1 20.8 6 23.4 4.21 11.4 0.27 14.8 41.427.9 0.98

In the case of Examples 3 and 5, the effluents of the test are collectedfor 4 hours in an ice-trap. 2 analyses by chromatography coupled with amass spectrometer are carried out per sample.

5 main products are detected per sample: acetone, water, acetic acid,propionic acid and acrylic acid.

The molar ratios propionic acid/acrylic acid are thus calculated foreach sample, for reaction temperatures of 320° C. and 360° C. Theaverage of the two analyses carried out per sample is given in Table 4below.

TABLE 4 Molar ratio Propionic acid/Acrylic acid Example 320° C. 360° C.3 6.74% 2.12% 5 6.43% 0.92%

It is noted that the molar ratio decreases with an increase in thetemperature. But also this ratio is still smaller for the catalystscontaining niobium.

Similarly as regards the acetone selectivity, this is even smaller whenthe catalyst contains niobium.

Example 7

In one example the catalyst prepared in Example 1 was tested again. Thiscatalyst has a particle size of less than 40 μm.

Then a reactor was loaded as shown in Example 2a) and operated accordingto the operating method shown in Example 2b).

i) Test E

In this test the oxidation of t propane is carried out at 400° C.

The duration of oxygen injection into the propane pulse is variedmaintaining constant propane and oxygen pressure.

The oxygen is injected at the end of the propane pulse to see if thereis an influence on the catalytic performances compared to an injectionat the start of the pulse.

The balance of 40 cycles is broken down as follows:

10 cycles of 30 s of propane+20 s of O₂ (the oxygen being injected atthe end of the propane pulse), with the proportions propane/O₂/He—Kr/H₂Oof 30/30/45/45, with a flow of helium-krypton of 4.27 N1/h.

There is then an intermediate pulse composed only of the flow of carriergas He—Kr/H₂O of 60 s, then a pulse of O₂ with the proportionsO₂/He—Kr/H₂O=20/45/45 for 60 s and another pulse of carrier gas of 60 s.

Then there is another series of 10 cycles of 30 s of propane+15 s ofoxygen with the proportions propane/O₂/He—Kr/H₂O of 30/30/45/45, with ahelium-krypton flow of 4.27 N1/h. Then there is an intermediate pulsecomposed only of the carrier gas He—Kr/H₂O of 60 s, then an oxygen pulsewith the proportions O₂/He—Kr/H₂O=20/45/45, for 60 s and anotherintermediate pulse of carrier gas of 60 s.Then there is another series of 10 cycles of 30 s of propane+10 s of O₂with the proportions propane/O₂/He—Kr/H₂O of 30/30/45/45, with ahelium-krypton flow of 4.27 N1/h. Then there is an intermediate pulsecomposed only of the flow of carrier gas He—Kr/H₂O of 60 s, then a pulseof O₂ with the proportions O₂/He—Kr/H₂O=20/45/45, for 60 s and anotherintermediate pulse of carrier gas of 60 s.Then there is another series of 10 cycles of 30 s of propane+5 s of O₂with the proportions propane/O₂/He—Kr/H₂O of 30/30/45/45, with ahelium-krypton flow of 4.27 N1/h. Then there is an intermediate pulsecomposed only of the flow of carrier gas He—Kr/H₂O of 60 s, then a pulseof O₂ with the proportions O₂/He—Kr/H₂O=20/45/45, for 60 s and anotherintermediate pulse of carrier gas of 60 s.ii) Test F

In this test, the oxidation of the propane is also carried out at 400°C.

The effect of the oxygen injection is compared at the end and at thestart of the propane pulse maintaining constant propane and oxygenpressures but also a constant duration of oxygen injection in thepropane pulse.

The balance of 40 cycles is broken down as follows:

10 cycles of 30 s of propane+20 s of O₂ (the oxygen being injected atthe end of the propane pulse), with the proportions propane/O₂/He—Kr/H₂Oof 30/30/45/45, with a helium-krypton flow of 4.27 N1/h. Then there isan intermediate pulse composed only of the flow of carrier gas He/Kr/H₂Oof 60 s, then an intermediate pulse of O₂ with the proportionsO₂/He—Kr/H₂O=20/45/45 for 60 s and another intermediate pulse of carriergas of 60 s.Then there is another series of 10 cycles of 30 s of propane+20 s ofoxygen (O₂ being injected at the end of the propane pulse) with theproportions propane/O₂/He—Kr/H₂O of 30/30/45/45, with a helium-kryptonflow of 4.27 N1/h. Then there is an intermediate pulse composed only ofthe flow of carrier gas He—Kr/H₂O of 60 s, then an intermediate pulse ofoxygen with the proportions O₂/He—Kr/H₂O=20/45/45, for 60 s and anotherintermediate pulse of carrier gas of 60 s.10 cycles of 30 s of propane+20 s of O₂ (the oxygen being injected atthe start of the propane pulse), with the proportionspropane/O₂/He—Kr/H₂O of 30/30/45/45, with a helium-krypton flow of 4.27N1/h. Then there is an intermediate pulse composed only of the flow ofcarrier gas He—Kr/H₂O of 60 s, then an intermediate pulse of O₂ with theproportions O₂/He—Kr/H₂O=20/45/45, for 60 s and another intermediatepulse of carrier gas of 60 s.Then there is another series of 10 cycles of 30 s of propane+20 s of O₂(the oxygen being injected at the start of the propane pulse), with theproportions propane/O₂/He—Kr/H₂O of 30/30/45/45, with a helium-kryptonflow of 4.27 N1/h. Then there is an intermediate pulse composed only ofthe flow of carrier gas He—Kr/H₂O of 60 s, then an intermediate pulse ofO₂ with the proportions O₂/He—Kr/H₂O=20/45/45, for 60 s and anotherintermediate pulse of carrier gas of 60 s.iii) Test GThis test was carried out in a similar way to test F.iv) Results of the tests

TABLE 5 Test E G F Conditions in 30 + 30/45/45 30 + 30/45/45 30 +30/45/45 the reaction: propane/He—Kr/H₂O Conditions in 20/45/45 20/45/4520/45/45 regeneration O₂/He—Kr/H₂O Comments Variation in the duration ofthe Duration of O₂ injection into the Duration of O₂ injection into theinjection of oxygen in the propane constant propane pulse injection ofconstant propane pulse injection of pulse. Injection of oxygen at the O₂at the end of the pulse then at O₂ at the end of the pulse then at endof the pulse the start of the pulse the start of the pulseRecapitulative Flask 1 Flask 2 Flask 3 Flask 4 Flask 1 Flask 2 Flask 3Flask 4 Flask 1 Flask 2 Flask 3 Flask 4 Number of CYCLES 10 10 10 10 1010 10 10 10 10 10 10 Duration of the 33.2 33.9 32.8 33.5 33.9 34.2 34.733.8 34.3 35.1 35.2 34.9 propane injection Duration of the 20 15 10 5 2020 20 20 20 20 20 20 oxygen pulses injected into the propane Yields (%)Acetaldehyde 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.00Propyl aldehyde 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Acetone 0.06 0.07 0.09 0.10 0.06 0.07 0.06 0.06 0.06 0.07 0.06 0.06Acrolein 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02Allyl alcohol 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Allyl acrylate 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 Acetic acid 1.62 1.93 2.47 2.75 2.02 2.07 2.12 2.13 2.40 2.552.36 2.49 Propionic acid 0.05 0.05 0.06 0.06 0.04 0.05 0.05 0.05 0.050.05 0.05 0.05 Acrylic acid 12.56 10.73 7.80 6.17 12.40 12.31 12.1112.47 11.28 11.60 11.30 11.74 Carbon monoxide 2.78 2.64 2.74 2.63 3.793.14 3.56 3.71 4.11 3.94 4.28 4.28 Carbon dioxide 2.37 2.74 3.43 3.713.64 2.95 3.84 3.98 3.81 3.74 4.37 4.43 Propylene 3.63 3.43 3.42 3.503.40 3.48 3.57 3.52 3.36 3.41 3.52 3.49 Propane 77.25 78.57 79.75 81.1674.80 76.16 74.96 74.41 74.97 74.82 74.13 73.71 Carbon balance (%) 100.3100.2 99.8 100.1 100.2 100.3 100.3 100.3 100.1 100.2 100.1 100.3

TABLE 6 Test E G F Conditions in 30 + 30/45/45 30 + 30/45/45 30 +30/45/45 the reaction: propane + oxygen/ He—Kr/H₂O Conditions in20/45/45 20/45/45 20/45/45 regeneration O₂/He—Kr/H₂O Comments Variationin the duration of the Duration of O₂ injection into the Duration of O₂injection into the injection of O₂ in the propane pulse. constantpropane pulse injection of constant propane pulse injection of Injectionof O₂ at the end of the pulse O₂ at the end of the pulse then O₂ at theend of the pulse then at the start of the pulse at the start of thepulse Recapitulative Flask 1 Flask 2 Flask 3 Flask 4 Flask 1 Flask 2Flask 3 Flask 4 Flask 1 Flask 2 Flask 3 Flask 4 Number of cycles 10 1010 10 10 10 10 10 10 10 10 10 Duration of the 33.2 33.9 32.8 33.5 33.934.2 34.7 33.8 34.3 35.1 35.2 34.9 propane injection Duration of theoxygen 20 15 10 5 20 20 20 20 20 20 20 20 pulses injected into thepropane Selectivities (%) Acetaldehyde 0.04 0.03 0.04 0.03 0.03 0.020.02 0.02 0.02 0.02 0.02 0.02 Propyl aldehyde 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 Acetone 0.25 0.32 0.45 0.52 0.25 0.290.24 0.23 0.26 0.28 0.21 0.22 Acrolein 0.09 0.09 0.08 0.08 0.08 0.080.07 0.07 0.07 0.07 0.06 0.07 Allyl alcohol 0.00 0.00 0.00 0.00 0.000.00 0.01 0.00 0.00 0.00 0.01 0.01 Allyl acrylate 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Acetic acid 7.01 8.93 12.3114.53 7.95 8.59 8.36 8.22 9.54 10.05 9.10 9.39 Propionic acid 0.22 0.240.28 0.32 0.17 0.22 0.19 0.19 0.19 0.21 0.18 0.19 Acrylic acid 54.3549.62 38.93 32.57 48.87 51.08 47.81 48.07 44.95 45.69 43.53 44.18 Carbonmonoxide 12.04 12.23 13.69 13.88 14.92 13.04 14.07 14.29 16.39 15.5116.49 16.11 Carbon dioxide 10.28 12.68 17.13 19.58 14.34 12.22 15.1515.35 15.19 14.73 16.83 16.68 Propylene 15.71 15.86 17.09 18.50 13.3914.46 14.08 13.57 13.39 13.43 13.56 13.14 Quantity of oxygen 3.58 3.533.35 3.31 4.37 4.00 4.47 4.49 4.46 4.57 4.83 4.89 consumed (g O/kgcatalyst) μmole propane for 1078 1101 1065 1088 1101 1111 1127 1098 11141140 1143 1133 1 cycle μmole O₂ 634 475 317 158 634 634 634 634 634 634634 634 added per cycle μmole O consumed 1120 1105 1049 1034 1368 12531399 1405 1396 1429 1511 1531 (products formed)/ cycle Propaneconversion 454 482 510 549 407 430 409 400 399 396 386 380 ratio (kgcatalyst/kg propane converted)It is seen that the catalytic performances of the catalyst are slightlybetter when the oxygen is injected at the end of the propane pulse. Theoxygen injected at this time allows reoxidizing of the already reducedcatalyst.By adding the oxygen to the propane pulse, conversion ratios of theorder of 380 to 400 kg/kg can be achieved with selectivities of acrylicacid of 45% and of propylene of 13% while having conversions of 26%.

Example 8 Preparation of the Precursors of Catalyst and Catalysts

I) Synthesis of the Precursor P1

This synthesis allows preparation of approximately 100 g of dryprecursor.

a) Preparation of the Niobium Solution

In a 500 ml beaker, the following are introduced:

-   -   88.0 g of demineralised water;    -   15.9 g of oxalic acid i.e. noxalate=0.126 moles    -   7.0 g of niobic acid i.e. nNb=0.042 moles giving an        oxalate/niobium ratio=3        The solution is stirred while heating to 60° C. for 2 hours.        However care is taken to not exceed a heating temperature of        80° C. in order to prevent the niobium oxalate formed from        decomposing. At the start the solution is milky. During heating        and stirring, the solution becomes less and less cloudy. It is        left to cool to approximately 30° C. in ambient air while        stirring continuously. In order to improve the limpidity, the        solution is centrifuged at 6200 r.p.m. for 12 minutes. A white        deposit of niobic acid is then observed.        b) Preparation of the Mo—V—Te Solution        In a 500 ml beaker, the following are introduced:    -   292.2 g of demineralised water;    -   19.1 g of telluric acid i.e. n_(Te)=0.083 moles    -   14.6 g of MVA i.e. n_(v)=0.125 moles    -   67.1 g of HMA i.e. n_(Mo)=0.380 moles        At first the solution is an orange/yellow colour and cloudy. The        mixture is stirred and heated to approximately 60° C. until a        limpid red colouring is observed (i.e. approximately 30        minutes). Then it is left to cool down to 30° C. at ambient        temperature while maintaining stirring.        c) Preparation of the Mo—V—Te—Nb—Si Gel        54.0 g of silica solution (i.e. n_(Si)=0.360 moles) is added to        the limpid red solution of Mo—V—Te obtained previously. The        solution becomes red/orange and thickens slightly. It is        maintained under stirring. Then the centrifuged solution of        niobium is introduced. A very rapid development of the mixture        is observed. In fact, in a few minutes the solution becomes        thicker and more orange. A gel forms. Attention must be paid to        the vortex during thickening of the mixture. This can diminish        and make the mixture even thicker. The stirring must then be        increased. Stir again for 45 minutes.        d) Drying of the Solution        The orange gel is poured into a teflon-coated plate then placed        in an oven at 130° C. overnight. A coarse brown/black powder is        then obtained which does not stick to the plate. The weight of        precursor obtained is 122.6 g. Depending on the quantities        introduced initially, the formula of the precursor is        MoV_(0.33)Te_(0.22)Nb_(0.11)        Si_(0.95)(Oxalate)_(0.33)(NH₄)_(1.19)O_(X).        Other precursors P2, P3 and P4        Table 7 gives the quantities of reagents introduced for the        preparation of 3 other precursors P2, P3 and P4. The same        operations as previously were carried out and the same changes        in colouring and appearance were observed for all these        experiments.

TABLE 7 Experiments P2 P3 P4 Solution oxalic acid mass (g) 15.9 15.915.9 of n_(oxalate) (mol) 0.126 0.126 0.126 niobium niobic acid mass (g)7.0 7.0 7.0 n_(Nb) (mol) 0.042 0.042 0.042 demineralised mass (g) 88.188.3 88.3 water Solution HMA mass (g) 67.2 67.1 67.1 of n_(Mo) (mol)0.381 0.380 0.380 MoV—Te MVA mass (g) 14.7 14.6 14.7 n_(v) (mol) 0.1260.125 0.126 telluric acid mass (g) 19.1 19.1 19.2 n_(Te) (mol) 0.0830.083 0.084 demineralised mass (g) 291.2 292.3 291.6 water Ludox silicemass (g) 54.7 54.1 54.1 n_(Si) (mol) 0.364 0.360 0.360

Precursors P2, P3 and P4 all have practically the same formula:

P2:MoV_(0.33)Te_(0.22)Nb_(0.11)Si_(0.96)(Oxalate)_(0.33)(NH₄)_(1.19)O_(x)

P3:MoV_(0.33)Te_(0.22)Nb_(0.11)Si_(0.95)(Oxalate)_(0.33)(NH₄)_(1.19)O_(x)

P4:MoV_(0.33)Te_(0.22)Nb_(0.11)Si_(0.95)(Oxalate)_(0.33)(NH₄)_(1.19)O_(x)

Loss by Combustion

This experiment aims to determine the quantity of niobic acid which didnot dissolve during preparation of the niobium oxalate solution. Duringpreparation of the precursor P4, the white deposit formed during thecentrifugation was sampled. This deposit has a weight of 0.5 g. It isplaced in a ceramic crucible. After evaporation in ambient air, thedeposit weighs 0.4 g. The deposit is then subjected to a rise intemperature of 3° C./min, from 20 to 600° C. and left at 600° C. for onehour.

After this heating, there remains only an Nb₂O₅ solid which weighs 0.2g. Thus there was a total loss of weight of 0.3 g.

Initially, 7.0 g of niobic acid at 79% or 7*0.79=5.53 g of Nb₂O₅ wasintroduced. Thus there was approximately 3.62% (=0.2/(7*0.79)*100) ofniobic acid which was not dissolved. Most of the niobic acid istherefore dissolved.

II) Precalcination Under Air

A precalcination under air is then carried out. The aim of this stage,among other things, is to eliminate the ammonia produced by the ammoniumheptamolybdate and the ammonium metavanadate. For this, 30 g ofprecursor is placed inside small steel cups and between two silica woolwads. Two parameters were adjusted during the operations:

-   -   the precalcination temperature: desired values 280; 300; 320 and        340° C.; and    -   the flow rates of air being introduced: desired values 0; 10;        30; 50 ml/min/g of catalyst.

Only the precalcination time remains the same for all the experiments.It is fixed at 4 hours.

III) Calcination Under Nitrogen

A calcination under nitrogen is then carried out. The flow rate, thetemperature and the calcination times are the same for all theoperations.

The conditions are as follows:

-   -   a flow rate of nitrogen of 50 ml/min/g of catalyst.    -   a calcination temperature of 600° C.    -   a calcination time of 2 hours.

Table 8 gives a summary of the precalcination and calcinationsconditions carried out on the different precursors.

TABLE 8 Flow rate of Temperature of Flow rate of air Temperature ofnitrogen (in Reference precalcination (in (in ml/min/g of calcination(in ml/min/g of of the Precursor ° C.) catalyst) ° C.) catalyst)calcinate P2 317 0 597 52.8 B1 316 11.1 596 48.8 B2 320 29.8 602 49.4 B3318 50.6 602 52.5 B4 P1 301 0 598 50.3 A1 300 11.0 597 49.0 A2 303 31.2600 49.5 A3 301 50.4 602 50.2 A4 P3 350 0 601 50.2 C1 347 9.9 598 51.1C2 349 30.7 601 50.4 C3 348 51.3 602 49.6 C4 P4 273 0 600 49.5 D1 27210.9 598 49.6 D2 273 31.9 600 49.9 D3 273 47.6 601 49.3 D4

Example 9

a) Apparatus

In order to simulate the method according to the invention, simulationswere carried out in the laboratory in a laboratory fixed bed reactor.

The following are loaded from the bottom to the top of a verticalreactor with cylindrical shape and made of pyrex:

-   -   a first height of 2 ml of silicon carbide in the form of        particles of 0.125 mm in diameter,    -   a second height of 5.00 g of catalyst in the form of particles        of 0.02 to 1 mm diluted with 10 ml of silicon carbide in the        form of particles of 0.125 mm in diameter,    -   a third height of 2 ml of silicon carbide in the form of        particles of 0.125 mm in diameter, and    -   a fourth height of silicon carbide in the form of particles of        1.19 mm in diameter, so as to fill all of the reactor.

b) Catalyst Tests

1) Operating Method

The reactor is heated to 250° C. and the vaporiser to 200° C. Theelectric initiation of the water pump is actuated. The flow rate ofHe—Kr is set at its nominal value of 4.25 N1/h.

Once the reactor and the vaporiser have reached the temperatures shownabove, the water pump is actuated. When the water is present at theoutlet of the reactor, the flowmeters of propane and oxygen areactuated. The reactor temperature is raised to the desired temperatureand left for 30 minutes so that the hot spot is stabilized.

A thermocouple is placed in the catalyst bed, the temperature of the hotspot can be read.

Then the measurements relating to the production of acrylic acid itselfcan be undertaken.

During each balance, liquid samples are taken. Gas samples are alsotaken in line.

Each gas-washing bottle with a capacity of 125 ml is filled with waterand when the flask is connected at the outlet of the reactor (when theliquid bubbles), the chronometer is started.

The liquid effluents are analyzed on an HP 6890 chromatograph, afterhaving carried out a specific calibration.

The gases are analyzed in line during the balancing on a Chrompackmicro-GC chromatograph.

An assay of the acidity is carried out on each flask to determine theexact number of moles of acid produced during each balance and tovalidate the chromatographic analyses.

2) Test of the Catalysts Prepared in Example 8

Catalysts A1 to A4, B1 to B4, C1 to C4 and D1 to D4 were tested at 3different temperatures: 380, 390 and 400° C.

The proportions of the constituents of the initial gaseous mixture wereas follows (in molar ratios):propane/oxygen/inert(He—Kr)/H₂O(vapour)=1/1/4.5/4.5 for a flow rate ofHe—Kr of 4.25 N1/h

The temperature giving the best results is 400° C.

The selectivities and yields obtained at this temperature are compiledin Tables 9 and 10 below. In these tables, the temperatures and the flowrates used in the precalcination of Example 8 as well as thetemperatures of the oven and of the hot spot during the catalyst testare also given.

The yields and selectivities obtained for allyl alcohol and allyacrylate are nil. Consequently they do not appear in Tables 9 and 10.

Table 9 shows the yields of carbon (TTU_(c)), with TTG_(c)=Σ TTU_(c) andTTG₀₂=Σ TTU₀, the acidities measured per assay with soda, the carbon andoxygen balances.

Table 10 shows the carbon selectivities.

TABLE 9 Catalyst D1 D2 D3 D4 A1 A2 A3 A4 Temperature of precalcination(° C.) 273 272 273 273 301 300 303 301 Air flow rate (ml/min/g ofcatalyst) 0 1091 31.9 47.57 0 11 31.2 50.4 Oven temperature (° C.) 400400 400 400 400 400 400 400 Hot spot temperature (° C.) 413.2 415.2412.5 412.6 409 414.4 408.3 407.8 Yields in Acetaldehyde 0.01 0.01 0.010.01 0.01 0.01 0.01 0 carbons Propanol 0 0 0 0 0 0 0 0 (TTU_(c) - %)Acetone 0.07 0.05 0.06 0.06 0.09 0.07 0.03 0.01 Acrolein 0.01 0.01 0.020.02 0.01 0.01 0.02 0.02 Acetic acid 0.77 0.79 0.76 0.78 0.87 1.01 0.250.1 Propionic acid 0.04 0.03 0.03 0.03 0.05 0.04 0.01 0.01 Acrylic acid6.15 15.66 14 13.71 8.35 13.17 3.3 1.18 CO 1.1 3.17 2.23 2.11 1.24 2.030.5 0.2 CO₂ 0.86 2.81 2.01 1.88 0.85 1.64 0.4 0.19 Propene 3.92 3.653.53 3.57 4.3 3.96 2.28 1.61 Propane 85.39 73.57 78.47 79.3 85.96 79.3992.4 95.53 Oxygen 73.69 38.21 49.72 53.12 71.76 54.66 88.13 95.6 Carbonbalance 98.3 99.7 101.1 101.5 101.7 101.3 99.2 98.8 Total carbon yield(TTG carbon) 12.9 26.2 22.6 22.2 15.8 22 6.8 3.3 Oxygen balance 97.898.2 99 100.8 101.1 100.5 100.2 100.6 Total oxygen yield (TTG_(m)) 24.160 49.3 47.7 29.4 45.8 12.1 5 Acidity measured (assay AB)* (ppm) 3051.96160.6 5210.4 5481.8 3546.4 5651.8 408.3 513.9 Catalyst B1 B2 B3 B4 C1C2 C3 C4 Temperature of precalcination (° C.) 317 316 320 318 350 347349 348 Air flow rate (ml/min/g of catalyst) 0 11.1 29.8 50.6 0 9.9230.74 51.31 Oven temperature (° C.) 400 400 400 400 400 400 400 400 Hotspot temperature (° C.) 412 416.3 408.58 407.6 411.9 408.2 406.1 405.4Yields in Acetaldehyde 0.01 0 0.01 0.01 0 0.01 0 0 carbons Propanol 0 00 0 0 0 0 0 (TTU_(c) - %) Acetone 0.09 0.07 0.04 0.04 0.05 0.02 0 0Acrolein 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.03 Acetic acid 0.95 0.950.36 0.31 0.58 0.28 0.03 0.01 Propionic acid 0.06 0.04 0.02 0.01 0.020.01 0 0 Acrylic acid 11.6 13.96 4.52 3.74 9.6 3.38 0.46 0.25 CO 1.382.01 0.74 0.55 1.43 0.71 0.16 0.15 CO₂ 1.11 1.55 0.58 0.45 1.07 0.720.29 0.26 Propene 4.13 4.01 3.09 2.37 3.53 2.2 1.15 0.81 Propane 79.6879.55 90.66 92.42 83.91 91.87 97.9 96.39 Oxygen 55.56 52.91 83.65 83.9766.54 85.12 97.03 97.68 Carbon balance 99 102.2 100.1 99.9 100.2 99.2100 97.9 Total carbon yield (TTG carbon) 19.3 22.6 9.4 7.5 16.3 7.4 2.11.5 Oxygen balance 94.6 99.7 100.6 97.5 99.2 99.7 100.6 100.5 Totaloxygen yield (TTG_(m)) 39 46.7 16.9 13.5 32.6 14.6 3.6 2.8 Aciditymeasured (assay AB)* (ppm) 4529.8 5960.5 2006.6 1552.6 3211.6 1364 210.189.1 *Assay A-B: acid-base assay of the number of moles of acid for soda

TABLE 10 Catalyst D1 D2 D3 D4 A1 A2 A3 A4 Temperature of 273 272 273 273301 300 303 301 precalcination (° C.) Air flow rate (ml/min/g of 0 109131.9 47.57 0 11 31.2 50.4 catalyst) Oven temperature (° C.) 400 400 400400 400 400 400 400 Hot spot temperature (° C.) 413.2 415.2 412.5 412.6409 414.1 408.3 407.8 Selectivities Acetaldehyde 0.04 0.04 0.03 0.040.04 0.04 0.13 0 in carbon Propanol 0 0 0 0 0 0 0 0 (TTU_(c) - %)Acetone 0.55 0.2 0.27 0.29 0.58 0.33 0.49 0.34 Acrolein 0.1 006 0.070.08 0.08 0.07 0.22 0.54 Acetic acid 5.94 3.01 3.37 3.52 5.5 4.62 3.693.13 Propionic 0.3 0.1 0.12 0.14 0.32 0.19 0.19 0.16 acid Acrylic acid47.5 59.8 61.8 61.8 53 60 48.5 35.6 CO 8.53 12.1 9.83 9.54 7.84 9.237.32 6.01 CO₂ 6.66 10.72 8.88 8.48 5.41 7.49 5.91 5.61 Propene 30.3313.95 15.6 16.09 27.28 18.04 33.57 48.57 Catalyst B1 B2 B3 B4 C1 C2 C3C4 Temperature of 317 316 320 318 350 47 349 348 precalcination (° C.)Air flow rate (ml/min/g of 0 11.1 29.8 50.6 0 9.92 30.74 51.31 catalyst)Oven temperature (° C.) 400 400 400 400 400 400 400 400 Hot spottemperature (° C.) 412 416.3 408.58 407.6 411.9 408.2 406.1 405.4Selectivities Acetaldehyde 0 0.02 0.14 0.08 0.03 0.14 0 0 in carbonPropanol 0 0 0 0 0 0 0 0 (TTU_(c) - %) Acetone 0.5 0.32 0.46 0.49 0.330.32 0 0 Acrolein 0.1 0.07 0.22 0.23 0.13 0.32 1.03 2.09 Acetic acid 4.94.21 3.87 4.09 3.57 3.81 1.34 0.87 Propionic 0.28 0.19 0.17 0.19 0.140.11 0 0 acid Acrylic acid 60 61.7 48.1 49.9 58.8 46 21.6 16.6 CO 7.138.87 7.92 7.37 8.78 9.64 7.38 9.82 CO₂ 5.73 6.85 6.19 5.95 6.56 9.7813.77 17.2 Propene 21.36 17.72 32.9 31.69 21.63 29.87 54.68 53.43

The conversion of the propane is generally considered as illustratingthe catalyst activity: the greater this conversion, the more active thecatalyst is. This conversion is given per TTG of carbon (TTG_(C)=ΣTTU_(C)).

It is noted that the conversion to propane is greater than 15% for thecatalysts precalcinated at 273° C. at flow rates of 10, 30 or 50ml/min/g of catalyst, at 300° C. and 320° C. at low flow rates and at350° C. at a no flow rate. It is also at these values that the highestacrylic acid yields (greater than 10%) and the greatest selectivitiesand the greatest acidities (greater than 3000 ppm) are obtained.

Generally, the TTG of oxygen (TTG₀₂) is also considered. As long as thisTTG remains below 60%, it is considered that there is still enoughoxygen which can serve to oxidize the propane. In this case, the maximumoxygen conversion capacities of the catalyst have not been achieved. Thereaction temperature can be raised in order to improve the conversion tooxygen. However it is preferable to prevent the oxygen conversion fromexceeding 80%.

The set of results relating to the selectivity and the yield of aceticacid is satisfactory.

It is thus noted that very good catalytic performances are obtained withseveral catalysts which have undergone very different precalcinationconditions:

-   -   at 273° C. under flow rates of air of 10, 30 and 50 ml/min/g of        catalyst;    -   at 300 and 320° C. under flow rates of air of 10 ml/min/g of        catalyst; and    -   350° C. under no air flow rate.

Example 10

Preparation of a Catalyst E

Catalyst E was prepared with the formula:MoV_(0.33)Te_(0.23)Nb_(0.11)Si₁O_(x) with the precursor:MoV_(0.33)Te_(0.23)Nb_(0.11) Si₁(Oxalate)_(0.33)(NH₄)_(1.19)O_(x) in themanner given in Example 8 carrying out a precalcination at 319° C.,under static air.

The drying which precedes the precalcination however has not beencarried out in the oven but using microwaves.

Example 11

Comparison of Catalyst E and Catalyst B2

Catalysts B and B2 were tested at 6 different temperatures, from 380° C.to 430° C. The results obtained are recorded in Tables 11 and 12 below.

TABLE 11 E B2 Temperature of 319 316 precalcination (° C.) Air flow rate(ml/min/g) 0 0 catalyst Oven temperature (° C.) 380 400 410 420 430 380400 410 420 430 Hot spot temperature (° C.) 389.6 414.3 426.8 439.9452.2 391.3 416.3 429.3 443 459.1 Yields in Acetaldehyde 0 0 0 0.01 0.710 0 0 0 0.48 carbon Propanol 0 0 0 0 0.01 0 0 0 0 0 TTU_(c) in Acetone0.12 0.06 0.04 0.03 0.12 0.14 0.07 0.06 0.04 0.12 %) Acrolein 0.01 0.010.02 0.02 0.03 0.01 0.02 0.02 0.02 0.03 Acetic acid 0.71 0.86 0.79 0.743.23 0.88 0.95 0.94 0.95 3.08 Propionic acid 0.08 0.04 0.03 0.02 2.950.09 0.04 0.03 0.02 0.96 Acrylic acid 9.88 16.1 18.7 20.87 6.63 9.5213.96 17 19.19 9.75 CO 0.82 1.71 2.6 3.98 6.76 0.98 2.01 3.09 4.8 7.26CO₂ 0.49 1.4 2.39 3.99 8.92 0.63 1.55 2.68 4.38 8.2 Propene 3.66 3.943.79 3.77 3.8 3.74 4.01 3.96 3.86 4.12 Propane 85.4 78.88 71.62 68.5565.74 86.1 79.55 73.41 68.38 66.67 Oxygen 71.14 48.49 34.54 18.77 0.0471.44 52.91 35.6 17.2 0.27 Carbon balance 101.2 103 100 102 98.9 102.1102.2 101.2 101.6 100.7 Total carbon yield (TTG) 15.8 24.1 28.4 33.433.1 16 22.6 27.8 33.3 34 Oxygen balance 99.7 97.5 96.6 97.8 95.3 10199.7 97.8 98.2 96.6 Total oxygen yield (TTG) 28.6 49 62 79.1 95.3 29.546.7 62.2 81 96.3 Acidity measured (assay 4216.7 6522.8 7207.2 01365580.5 3828.5 5960.5 6906.7 8610.2 5790.5 AB) (ppm)

TABLE 12 E B2 Temperature of 319 316 precalcination (° C.) Air flow rate(ml/min/g) 0 11.1 catalyst Oven temperature (° C.) 380 400 410 420 430380 400 410 420 430 Hot spot temperature (° C.) 389.6 414.3 426.8 439.9452.2 391.3 416.3 429.3 443 459.1 Yields in Acetaldehyde 0 0 0 0 2.13 00.02 0 0 1.41 carbon Propanol 0 0 0 0 0.03 0 0 0 0 0 TTU_(c) in Acetone0.8 0.26 0.2 0.1 0.35 0.9 0.32 0.2 0.1 0.35 %) Acrolein 0.1 0.05 0.1 0.10.08 0.1 0.07 0.1 0.1 0.08 Acetic acid 4.5 3.57 2.8 2.2 9.75 5.5 4.213.4 2.9 9.05 Propionic acid 0.5 0.18 0.1 0.06 8.89 0.5 0.19 0.1 0.062.83 Acrylic acid 62.6 66.7 65.9 62.4 20 59.6 61.7 61.2 57.7 28.7 CO5.17 7.08 9.17 11.9 20.4 6.14 8.87 11.11 14.43 21.36 CO₂ 3.12 5.8 8.4411.94 26.91 3.94 6.85 9.65 13.17 24.11 Propene 23.2 16.34 13.37 11.2811.46 23.37 17.72 14.26 11.6 12.13

It is noted that the behaviours are similar for these two catalysts.Therefore there is no difference a priori between drying with microwavesand drying in the oven.

The yield of acrylic acid and the acidity are greater as the reactiontemperature increases. These results show that the maximum yield ofacrylic acid is reached at 420° C.

1. Method for the production of acrylic acid from propane, in which agaseous mixture comprising propane, molecular oxygen, water vapour, andoptionally an inert gas is passed over a catalyst with the formula (I):Mo_(l)V_(a)Te_(b)Nb_(c)Si_(d)O_(x)  (I) in which: a is between 0.006 and1, inclusive; b is between 0.006 and 1, inclusive; c is between 0.006and 1, inclusive; d is between 0 and 3.5, inclusive; and x is thequantity of oxygen bound to the other elements and depends on theiroxidation state, in order to oxidize the propane to acrylic acid,wherein the molar ratio propane/molecular oxygen in the initial gaseousmixture is greater than or equal to 0.5.
 2. Method according to claim 1,in which the molar proportions of the constituents of the initialgaseous mixture are as follows:propane/O₂/inert gas/H₂O (vapour)=1/0.05-2/1-10/1-10.
 3. Methodaccording to claim 1, in which the molar proportions of the constituentsof the initial gaseous mixture are as follows:propane/O₂/inert gas/H₂O (vapour)=1/0.1-1/1-5/1-5.
 4. Method accordingto claim 1, in which, in the catalyst of formula (I): a is between 0.09and 0.8, inclusive; b is between 0.04 and 0.6, inclusive; c is between0.01 and 0.4, inclusive; and d is between 0.4 and 1.6, inclusive. 5.Method according to claim 1 , wherein the oxidation reactions arecarried out at a temperature of 200 to 500° C.
 6. Method according toclaim 1, wherein the oxidation reactions are carried out at atemperature of 250 to 450° C.
 7. Method according to claim 1, whereinthe oxidation reactions are carried out at a pressure of 1.01×10⁴ to1.01×10⁶ Pa (0.1 to 10 atmospheres).
 8. Method according to claim 1,wherein the oxidation reactions are carried out at a pressure of5.05×10⁴ to 5.05×10⁵ Pa (0.5-5 atmospheres).
 9. Method according toclaim 1, which is used until there is a reduction ratio of the catalystcomprised between 0.1 and 10 g of oxygen per kg of catalyst.
 10. Methodaccording to claim 1, wherein once the catalyst has at least partiallychanged to the reduced state, its regeneration is carried out accordingto reaction (3):SOLID_(reduced)+O₂→SOLID_(oxidized)  (3) by heating in the presence ofoxygen or a gas containing oxygen at a temperature of 250 to 500° C.,for a period necessary for the reoxidation of the catalyst.
 11. Methodaccording to claim 10, wherein the oxidation and the regeneration (3)reactions are carried out in a device with two stages, namely a reactorand a regenerator which operate simultaneously and in which two catalystloads alternate periodically.
 12. Method according to claim 10, whereinthe oxidation and the regeneration (3) reactions are carried out in thesame reactor alternating the periods of reaction and regeneration. 13.Method according to claim 10, wherein the oxidation and the regeneration(3) reactions are carried out in a reactor with a moving bed.
 14. Methodaccording to claim 1, in which: a) the initial gaseous mixture isintroduced into a first reactor with a moving catalyst bed, b) at outletof the first reactor, the gaseous mixture is separated from thecatalyst; c) the catalyst is returned into a regenerator; d) the gaseousmixture is introduced into a second reactor with a moving catalyst bed;e) at outlet of the second reactor, the gaseous mixture is separatedfrom the catalyst and acrylic acid contained in the separated gaseousmixture is recovered; f) the catalyst is returned into the regenerator;and g) regenerated catalyst from the regenerator is reintroduced intothe first and second reactors.
 15. Method according to claim 14, inwhich the first and second reactors are vertical and the catalyst ismoved upwards by the gas flow.
 16. Method according to claim 1, whereinthe oxidation reactions are carried out with a residence time of 0.01 to90 seconds in each reactor.
 17. Method according to claim 1, wherein theoxidation reactions are carried out with a residence time of 0.1 to 30seconds in each reactor.
 18. Method according to claim 1, whereinpropylene produced or the propane which has not reacted or both arerecycled to an inlet of a reactor, or if there are several reactors, toinlet of a first reactor.
 19. Method according to claim 1, in which areactor, or when there are several reactors, at least one of thereactors, also comprises a cocatalyst corresponding to the followingformula (II):Mo_(l)Bi_(a′)Fe_(b′)Co_(c′)Ni_(d′)K_(e′)Sb_(f′)Ti_(g′)Si_(h′)Ca_(1′)Nb_(j′)Te_(k′)Pb_(l′)W_(m′)Cu_(n′)  (II)in which: a′ is between 0.006 and 1, inclusive b′ is between 0 and 3.5,inclusive; c′ is between 0 and 3.5, inclusive; d′ is between 0 and 3.5,inclusive; e′ is between 0 and 1, inclusive; f′ is between 0 and 1,inclusive; g′ is between 0 and 1, inclusive; h′ is between 0 and 3.5,inclusive; i′ is between 0 and 1, inclusive; j′ is between 0 and 1,inclusive; k′ is between 0 and 1, inclusive; l′ is between 0 and 1,inclusive; m′ is between 0 and 1, inclusive; and n′ is between 0 and 1,inclusive.
 20. Method according to claim 19, in which the cocatalyst isregenerated and circulates, in the same way as the catalyst.
 21. Methodaccording to claim 19, in which, in the cocatalyst of formula (II): a′is between 0.01 and 0.4, inclusive; b′ is between 0.2 and 1.6,inclusive; c′ is between 0.3 and 1.6, inclusive; d′ is between 0.1 and0.6, inclusive; e′ is between 0.006 and 0.01, inclusive; f′ is between 0and 0.4, inclusive; g′ is between 0 and 0.4, inclusive; h′ is between0.01 and 1.6, inclusive i′ is between 0 and 0.4, inclusive; j′ isbetween 0 and 0.4, inclusive; k′ is between 0 and 0.4, inclusive; l′ isbetween 0 and 0.4, inclusive; m′ is between 0 and 0.4, inclusive; and n′is between 0 and 0.4, inclusive.
 22. Method according to claim 19, inwhich, a weight ratio of the catalyst to the cocatalyst greater than 0.5is used.
 23. Method according to claim 19, in which, a weight ratio ofthe catalyst to the cocatalyst of at least 1 is used.
 24. Methodaccording to claim 19, in which the catalyst and the cocatalyst aremixed.
 25. Method according to claim 19, in which the catalyst and thecocatalyst are present in the form of pellets, each pellet comprisingboth the catalyst and the cocatalyst.
 26. Method according to claim 1,comprising repetition, in a reactor provided with the catalyst offormula (I) defined in claim 1, or the cocatalyst of formula (II)defined in claim 19, of a cycle comprising the following successivestages: 1) a stage of injection of the gaseous mixture as defined inclaim 1; 2) a stage of injection of water vapour or inert gas; 3) astage of injection of a mixture of molecular oxygen, water vapour orinert gas; and 4) a stage of injection of water vapour or inert gas. 27.Method according to claim 26, wherein the cycle comprises an additionalstage which precedes or follows stage 1) and during which a gaseousmixture corresponding to that of stage 1) but without molecular oxygenis injected, the molar ratio propane/molecular oxygen then beingcalculated globally for stage 1) and this additional stage.
 28. Methodaccording to claim 27, wherein the additional stage precedes stage I) inthe cycle.
 29. Method according to claim 27, wherein the reactor is areactor with a moving bed.
 30. Method for the production of acrylic acidfrom propane, in which a gaseous mixture comprising propane, molecularoxygen, water vapour, and optionally an inert gas is passed over acatalyst with the formula (I):Mo_(l)V_(a)Te_(b)Nb_(c)Si_(d)O_(x)  (I) in which: a is between 0.006 and1, inclusive; b is between 0.006 and 1, inclusive; c is between 0.006and 1, inclusive; d is between 0 and 3.5, inclusive; and x is thequantity of oxygen bound to the other elements and depends on theiroxidation state, in order to oxidize the propane to acrylic acid,wherein the molar ratio propane/molecular oxygen in the initial gaseousmixture is greater than or equal to 0.5, and in which a) the initialgaseous mixture is introduced into a first reactor with a movingcatalyst bed, b) at the outlet of the first reactor, the gases areseparated from the catalyst; c) the catalyst is returned into aregenerator; d) the gases are introduced into a second reactor with amoving catalyst bed; e) at the outlet of the second reactor, the gasesare separated from the catalyst and the acrylic acid contained in theseparated gases is recovered; f) the catalyst is returned into theregenerator; and g) the regenerated catalyst from the regenerator isreintroduced into the first and second reactors.
 31. Method according toclaim 30, in which the molar proportions of the constituents of theinitial gaseous mixture are as follows:propane/O₂/inert gas/H₂O (vapour)=1/0.05-2/1-10/1-10.
 32. Methodaccording to claim 30, in which, in the catalyst of formula (I): a isbetween 0.09 and 0.8, inclusive; b is between 0.04 and 0.6, inclusive; cis between 0.01 and 0.4, inclusive; and d is between 0.4 and 1.6,inclusive.
 33. Method for the production of acrylic acid from propane,in which a gaseous mixture comprising propane, molecular oxygen, watervapour, and optionally an inert gas is passed over a catalyst withformula (I):Mo_(l)V_(a)Te_(b)Nb_(c)Si_(d)O_(x)  (I) in which: a is between 0.006 and1, inclusive; b is between 0.006 and 1, inclusive; c is between 0.006and 1, inclusive; d is between 0 and 3.5, inclusive; and x is thequantity of oxygen bound to the other elements and depends on theiroxidation state, in order to oxidize the propane to acrylic acid,wherein the molar ratio propane/molecular oxygen in the initial gaseousmixture is greater than or equal to 0.5, comprising repetition, in areactor provided with the catalyst of formula (I) defined above, of acycle comprising the following successive stages: 1) a stage ofinjection of the gaseous mixture as defined above; 2) a stage ofinjection of water vapour or inert gas; 3) a stage of injection of amixture of molecular oxygen, water vapour or inert gas; and 4) a stageof injection of water vapour or inert gas.
 34. Method according to claim33, in which, in the catalyst of formula (I): a is between 0.09 and 0.8,inclusive; b is between 0.04 and 0.6, inclusive; c is between 0.01 and0.4, inclusive; and d is between 0.4 and 1.6, inclusive.
 35. Methodaccording to claim 33, wherein the cycle comprises an additional stagewhich precedes or follows stage 1) and during which a gaseous mixturecorresponding to that of stage 1) but without molecular oxygen isinjected, the molar ratio propane/molecular oxygen then being calculatedglobally for stage 1) and this additional stage.
 36. Method according toclaim 35, wherein the additional stage precedes stage I) in the cycle.